A laser oscillator of the kind mentioned comprises a laser medium of which the emission band is so broad that the laser oscillator is tunable by means of a wavelength selective member.
The fundamental structure of tunable laser oscillators is known, for instance, from DE 29 18 863 C2. Such laser oscillators comprise a broadband emitting laser medium, for example a dye solution, excimers obtained by gas discharge, or solid state materials. For wavelength tuning, a dispersing member is arranged inside the resonator of such a laser oscillator. This tuning member (wavelength selective member), for example, may be a grating or a dispersing prismatic arrangement. Other wavelength selective means may be used as well, such as an etalon, a Fabry-Perot interferometer, or double refractive crystals.
In the case of the arrangement disclosed in DE 29 18 863 C2, the laser radiation uncoupled contains both narrowband radiation whose spectral distribution is determined substantially by the beam expanding device and the wavelength selective member and relatively broadband radiation whose spectral distribution is given essentially by the broadband emitting laser medium. This broadband spontaneous radiation is referred to as ASE (amplified spontaneous emission). The ratio between the energy of the narrowband laser radiation and the energy of the ASE is called spectral purity of the radiation issuing from the laser oscillator. This spectral purity of the laser output radiation deteriorates, for example, if the laser medium is excited in pulsed fashion and the duration of the excitation pulses lies in the same order of magnitude as the transit time of the light inside the laser resonator (oscillator). Typically, such transit time is some 2 to 3 nanoseconds, depending on the type of laser.
The spectral purity of laser radiation also deteriorates, for instance, if the output radiation is intensified still further as soon as the ASE and the spectrally pure laser light (i.e. the radiation having the relatively narrow bandwidth) take different courses in time inside the laser. In general, ASE occurs prior to laser radiation.
DE 29 18 863 C2, already mentioned above, also teaches to connect a means for spectral filtering downstream of the laser resonator to suppress the ASE and improve the spectral purity of the output radiation.
German patent application P 43 02 378 discloses a laser oscillator of the generic type in question with which the spectral purity of the pulsed output beam is improved in that the beam emitted by the laser oscillator is coupled out from the resonator by means of an optical element after having passed through an expansion means and the wavelength selective member and before again passing through the laser medium where once more broadband radiation (spontaneous fluorescence and ASE) is mixed to it. Expensive optical means are needed for this earlier proposal, particularly expander means and complicated optics for coupling out the beam.